The present invention relates to a method of manufacturing a thin steel sheet of high carbon steel, and particularly to a method of manufacturing a carbon steel sheet which has a very fine crystal grain structure after heat treatment and which is superior to a conventional thin steel sheet with respect to resistance to mechanical shock and wear, and which can successfully prevent cracking caused by hydrogen infiltrated into the steel during use.
The steel sheet of the present invention is easy to produce and easy to work, and therefore, it is suitable for forming a variety of articles including chain elements, gear members, clutch members, buckles for seat belts, and washers.
In general, such articles are manufactured from a high carbon steel such as S30CM, S70CM, SK7M, or SK4M and a low alloy high carbon steel such as SCM435 or SCM445 as specified in JIS G 3311. These high carbon steels and low alloy, high carbon steels are, if necessary after hot rolling and descaling by pickling, subjected to cold rolling with a certain degree of reduction in area as well as spheroidizing by heating the steel at a temperature of around the Ac.sub.1 point for an extended period of time in order to improve the accuracy of the thickness of the sheet and formability including its suitability for blanking, bending, and pressing which are usually carried out by customers. After forming into a final shape, heat treatment such as quenching and tempering, austempering, and the like is performed to harden the article being manufactured, resulting in improvement in the resistance to wear and mechanical shock.
Therefore, steel sheet for use in manufacturing these articles must exhibit a relatively high degree of strength as well as shock- and wear-resistant properties only after heat treatment. For this purpose it is necessary to employ steel with a high carbon content. Since the shock- and wear-resistant properties of the final articles are influenced by heat treatment conditions, and especially the tempering temperature, steel sheet "as quenched" or steel sheet "tempered at 650.degree. C. or lower" (usually 180.degree.-450.degree. C.) in the case of quenched material, or steel sheet austempered at a temperature of 500.degree. C. or lower (usually 200.degree.-450.degree. C.) in the case of austempered material are carefully chosen depending on the properties required for the use.
However, since thin steel sheet of high carbon steel, especially that with a higher content of carbon suffers from an increase in strains in the steel and precipitation of a large amount of carbides in the course of heat treatment, it is inevitable that the shock-resistance as well as the resistance to hydrogen embrittlement are insufficient, even if the heat treatment conditions are determined carefully.